The invention relates to a graphite tube atomizer for flameless atomic absorption spectroscopy which includes a support for a graphite tube having two annular contact pieces disposed to permit passage of a measuring beam along the axis of the graphite tube. The contact pieces are electrically connected to current conducting cables and mounted on a relatively moveable, mechanically biased, cooled housing portions.
One known graphite tube atomizer is described, for example, in U.S. patent application Ser. No. 608,558 which was filed on Aug. 28, 1975 now U.S. Pat. No. 4,022,530. The graphite tube atomizer theredescribed has two housing portions each of which consist of a cooling jacket as well as a socket both of which are mounted in an adjustable manner on a base. A contact piece is supported inside the cooling jacket and has a coolant passage within it. The cooling jacket of one housing portion is axially moveable relative to the socket and biased towards the other housing portion by means of a pair of annular cup springs. The contact pieces and the cooling jackets are also annular so as to form a center aperture for the passage of a measuring beam. A graphite tube is inserted between the contact pieces by pushing one cooling jacket with the associated contact against the action of the annular cup springs thereby permitting the graphite tube to be interposed between the contact pieces. When the cooling jacket is released, the graphite tube is held resiliently between the contact pieces by the bias provided by the annular cup springs. The cooling jackets and contact pieces are mounted in electrically insulating manner and are connected to a current supply. The bias provided by the annular cup spring provides contact pressure between the contact pieces and the graphite tube thus providing a low resistance contact so that heating current can pass through the contact pieces and the graphite tube. In operation, a sample to be tested is introduced into the graphite tube through a radial bore. The sample is decomposed and atomized by directing a high current through the graphite tube. The absorbtion to which the measuring beam passing along the axis of the graphite tube is subjected in the "atomic cloud" thus formed serves as a measure of a quantity of the element being looked for in the sample.
The above described atomic absorption apparatus accommodates varying length graphite tubes as the socket of one housing portion is axially displaceable on a pair of guide rods relative to the other housing portion. If a shorter tube is installed in the apparatus, the socket is accordingly displaced to accommodate its shorter length.
In prior art arrangements of the type described above, consecutive analyses are usually carried out in the device using a single graphite tube. After each analysis, the graphite tube is heated and the formed atomic cloud is flushed from the tube by an inert gas flow therethrough. This inert gas flow protects the graphite tube from oxygen and prevents combustion of the tube itself. Thereafter, the next sample is fed through the radial bore of the graphite tube. Exchange of graphite tubes is only required after a rather large number of analyses when the tube has become worn in spite of the inert gas flow which tends to prevent such wear.
Slight length differences between graphite tubes do occur and can be compensated for by the yielding of the annular cup springs. The same applies to the thermal dilation of the graphite tube. However, the spring bias of the annular cup springs and thus the contact pressure between the contact pieces and the graphite tube is dependent on the length of the graphite tube and on the thermal dilation thereof. Accordingly, the apparatus described above is subject to some variation as a function of tube length and the thermal dilation which occurs.
The above described apparatus is reasonably satisfactory in environments where graphite tube exchange is not required, however, there are applications where such graphite tube changes are required frequently. An example of such an application is in analyzing fine dusts in gases by means of atomic absorption spectroscopy. In such an application, the fine dust contained in the gas is deposited electrostaticly directly in a graphite tube. The graphite tube is then placed in a graphite tube atomizer and subsequently heated until the dust particles are atomized (see German Patent Specification No. 24 01 873, German Patent Specification No. 24 35 091). In such applications or similar applications, the graphite tube is exchanged for each analysis. For flameless atomic absorbtion spectrometers of the type described in the above pending U.S. patent application Ser. No. 608,558, even the aid of a special noncontaminating tool is of little help in placing a graphite tube into the atomizer because its physical design makes replacing the graphite tube quite difficult.
In applications where frequent exchange of graphite tubes is required, it is quite common that the length of the graphite tubes does vary somewhat. When utilized in an apparatus of the type described in the above mentioned U.S. patent application, Ser. No. 608,558, different contact pressures occur between tests due to differing tube length so that some tests may be made with inappropriate electrical contact resistance between the contacts and the end of the graphite tube. Other times, the bias of the annular cup springs is too large which runs the risk of damaging the fragile graphite tubes.
In view of the foregoing difficulties, it is the primary objective of the invention to provide a graphite tube atomizer for flameless atomic absorption spectroscopy wherein the tube is held in a mechanism which permits easy tube changing without giving rises to the problems associated with prior art graphite tube atomizers.
It is still a further objective of the invention to provide a graphite tube atomizer for atomic absorption spectroscopy which has a tube holding means including contacts which are urged towards opposite ends of the tube and maintained in constant pressure contact with the tube.
In achieving these and other objectives of the invention, the graphite tube atomizer of the present invention is designed so that exchanging the graphite tube is facilitated as compared to the approach for changing graphite tubes in prior art mechanisms. In particular, it is possible to place the graphite tube into the tube atomizer without contamination. Furthermore, a well-defined contact pressure between the graphite tube and contact pieces is insured independently of the graphite tube length.
The stated objectives of the invention are achieved by a graphite tube support mechanism including a reversable drive means which is operable to move the housing portions away from each other in one driving direction to permit removal of the graphite tube and moveable towards each other in the other driving direction to permit a graphite tube to be interposed therebetween. In addition, the drive means provides contact pressure between the contact pieces and the graphite tube which is selectable regardless of the tube length thereby avoiding the problem of high resistance contact as well as the problem of excessive pressure on the fragile tube.
The drive means in its preferred form is a pneumatic cylinder or other drive means capable of selective movement in opposite directions and exerting a selectable force. Preferably, the first housing portion of the graphite tube atomizer is stationary and the second housing portion is moveable relative thereto. When the drive means is a pneumatic cylinder, it is attached to the first housing portion with the double-acting piston being coupled to the second housing portion. Then, the force with which the contact pieces are pressed against the graphite tube depends only on the pressure in the pneumatic cylinder and is independent of tube length or the position of the housing portions which encircle the graphite tube.